DC High-Potential Test

The DC hi-pot test is applied at above the rated voltage of a transformer to evaluate the condition of winding insulation. The DC high-voltage test is not recommended on power transformers above 34.5 kV; instead the AC hi-pot test should be used. 
Generally, for routine maintenance of transformers, this test is not employed because of the possibility of damage to the winding insulation. However, this test is made for acceptance and after repair of transformers.
If the hi-pot test is to be conducted for routine maintenance, the AC test values should not exceed 65% of factory AC test value. The routine maintenance AC voltage value should be converted to an equivalent DC voltage value by multiplying it by 1.6, that is, 1.6 times the AC value for periodic testing (i.e., 1.6 × 65 = 104% of AC factory test value). The DC hi-pot test can be applied as a step-voltage test where readings of leakage current are taken for each step. If excessive leakage current is noticed, voltage can be backed off before further damage takes place. For this reason, the DC hi-pot test is considered to be a nondestructive test.
 Some companies conduct the AC hi-pot test at rated voltage for 3 min for periodic testing instead of the 65% of factory test voltage. The hi-pot test values for DC voltages are shown in Table 1.1.
The procedure for conducting this test is as follows (refer to Figure 1.1a and b for test connections):

FIGURE 1.1
Transformer high voltage (hi-pot) test connection: (a) high winding hi-pot test connection and
(b) low winding hi-pot test connections. 


Transformer must have passed the insulation resistance test immediately
prior to starting this test.
• Make sure transformer case and core are grounded.
• Disconnect all high-voltage, low-voltage, and neutral connections,
low-voltage control systems, fan systems, and meters connected to the transformer winding and core.
• Short-circuit with jumpers together all high-voltage bushings and all low-voltage bushings to ground as discussed under “Insulation resistance measurements.

Connect hi-pot test set between high-voltage winding and ground.
Gradually increase test voltage to the desired value. Allow test voltage duration of 1 min, after which gradually decrease voltage to zero.
• Remove low-voltage to ground jumper and connect hi-pot test set
between low-voltage winding and ground. Also connect the shortcircuited
high-voltage winding to ground. Gradually increase test voltage to desired value. Allow the test voltage duration of 1 min, after which gradually decrease voltage to zero.
• If the preceding two tests do not produce breakdowns or failures, the transformer is considered satisfactory and can be energized.
• Remove all jumpers and reconnect primary and secondary connections and other system equipment that may have been disconnected.
The following are some cautions and considerations in performing hi-pot
tests:
In liquid-filled transformers two insulation systems are in series, that is, solid insulation with oil or synthetic fluid. When AC or DC hi-pot test voltage is applied, the voltage drops are distributed as follows:

 Table 1.1
 

When using DC hi-pot test voltage on liquid-filled transformers, the solid insulation may be overstressed.
Insulation that may be weakened near the neutral may remain in service due to lower stress under operating conditions. However, when subjected to hi-pot test voltage, it may break down and require immediate repair. The weakened insulation may usually be detected by the measurement at lower voltages.
If a hi-pot test is to be conducted for routine maintenance, consider the following in advance: (1) assume that a breakdown will occur, (2) have replacement or parts on hand, (3) have personnel available to perform work, and (4) is the loss of the transformer until repairs are made beyond the original routine outage.